Projection lamp



y 9 E.. w. KELLOGG 2,080,278

. PROJECTION LAMP Filed Aug. 15, 1934 l I 4 INVENTO/EZ. W W Emamwjfzi/lqyy,

14 18 ggwym MTTOB/VEK Patented May 11, 1937' PROJECTION LAMP Edward W. Kellogg, Moorestown, N. ,L, assign'or, by mesne assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application August 15, 1934, Serial No. 739,880

12 Claims.

This invention relates to projection lamps, and more particularly to a lamp adapted for use in sound recording and reproducing apparatus employing photographic sound records, and for other purposes for which high intrinsic brilliancy as well as freedom from rapid fluctuations is desired.

In making a photographic record of sound, fluctuations in the source cause irregularities in the exposure of different parts of the film, and these irregularities result in noise when the record is reproduced. Likewise, fluctuations in the light source employed in the reproducing equipment result in noise. Fluctuations which, owing to the persistence of vision, are not objectionable for strictly visual purposes, may be very objectionable in such lamp applications as I have mentioned. For this reason, arc lamps are not satisfactory, and the only lamps which have been found entirely suitable for such purposes have been those employing incandescent filaments, op-

erated either on direct current or on alternating current of niuch higherfrequency than the usual 50 to 60 cyc es per second. i

Operation of incandescent lamps on alternating current results in temperature fluctuation at double the frequency of the current alternations. The higher the frequency the smaller are the fluctuations in temperature and consequently in luminosity. Likewise, for a given frequency, the

' heavier the filament wire the smaller are fluctuations in temperature.

This is because the thermal energy stored in the hot metal is greater for a given rate of. energy radiation in the heavy,

filament wire than in a'thin wire wherein the ratio of radiating surface to cross section of wire is greater than in a heavy wire.

Whererequirements in regardto freedom from fluctuation are not extremely rigid, low voltage, high current lamps operated on 60 cycle alternating current have been employed, and by going to sufllciently heavy filaments the fluctuations can be reduced to any required degree. However, it has not proved practical to go far enough in this direction to give really satisfactory operation. Ii. the filament is designed to operate at less than about six volts, it becomes so short that an excessive fraction of the heat generated within it is carried by metallic heat conduction to the supporting wires. with a minimum practical voltage, high current necessarily means 'high wattage, and this calls for large bulbs and'more ventilation. Were another material having the high boiling and melting points of tungsten but having much higher resistivity available, the desired thick filament lamp might be designed, but in the absence of such materials, it appears that the only solution is in supplying the power in some other manner than by ohmic resistance to the flow of current through the metal. 5

Among the lamps heretofore proposed for pic- 5 ture projection purposes, there are some in which the light radiating body is heated by electron bombardment instead of by passage of current. This permits the radiating body to be made in the form of a small, compact head or button of tungsten instead of an elongated conductor, and such a button will store enough heat to reduce temperature fluctuations to so small a magnitude that they are not objectionable. A lamp of this type, if exhausted to high vacuum, would, for a made positive with respect to the cathode. The

lamp could be operated on either direct or alternating current, and in the latter case would rectify its own current. A high vacuum lamp, however, is open to the objection that the part of the bulb nearest the light radiating body becomes blackened in a very short time. The presence of an inert gas in sufllcient pressure not only reduces the evaporation of the tungsten from the hot electrode, but, by convection, carries the tungsten vapor to the top of the bulb instead of permitting it to travel straight to the nearest glass surface. In view of this important advantage of operating high intensity lamps in a gas, such lamps of the type 'just described as have found any considerable use are of the gas filled rather than of the high vacuum type. The presence of the gas, however, changes the mode ofconduction between the cathode and the anode from what would be a pure electron stream in a high vacuum lamp to a luminous are through body is heated by electron bombardment which 55 will be suitable for we in sound recording and reproducing apparatus.

A further object of my invention is to provide an improved projection lamp of the type set forth wherein the arc is so confined that it will not interfere with or contribute anything to the light emitted by the lamp in the useful direction.

Still a further objectof my invention is to provide a projection lamp of the type set forth which can be operated on either D. C. or A. C., which will provide a high intensity source, and which will not give rise to hum when used for sound recording or reproducing purposes. I

Aside from the fact that the lamps now practically available for sound recording and reproducing purposes cannot be satisfactorily operated on alternating current owing to cyclic temperature fluctuations of the filament, the present lamps have several other drawbacks. Optical systems for use in sound equipment utilize, in general, only so much of the light emitted'by the source as can be collected by a condensing lens. The light which reaches the sensitive element, whether it be film or photocell, is further restricted by apertures which, from one or another design consideration, are restricted to small size. Usually (and this involves placing the condenser lens close to the'source) the image of the source can be made large .enough to more than cover or fill the aforementioned small aperture. When this condition is met, there is no gain in useful light by employing a larger source. Increase in the amount of useful light is possibleonly by increasing the concentration of the light, or, in other words, by employing a source which emits more light from a given superficial or projected area. This can be done in several ways, all of which may be realized in my invention, by using a solid or continuous source, by raising the temperature, and by increasing the emissivity. The spaces between the turns of a coiled filament obviously contribute nothing to the light radiated in a given direction. A solid block of tungsten of the same outside dimensions and at the same temperature would radiate more light, and the lamp which is the subject of the present invention utilizes such a solid block as its radiating element. The thick block of tungsten can also be worked at a higher temperature than a slender filament without causing short life due to a burn-out. Polished tungsten emits much less light per unit-of surface area than a black body at the same temperature. If, however, the surface ismade very rough, as, for example, by numerous closely spaced relatively deeppits, the

emissivity of the tungsten can be increased so as to approach that of a black body. The same object is attained in some measure by a single pit or concavity. The interior surfaces of thepits transmit light in the useful direction not only by virtue of their own temperature, but also by reflecting light which they receive from other nearthe image of the source, as produced by the condenser or other lenses, shall be as nearly as possible uniformly bright over the entire area utilized. This can obviously not be attained with lamps of the coiled filament type, but can be realized with a lamp of the type involved in my invention. ,For such special applications the roughening or pitting would be either dispensed with, leaving a plane uniform surface, or a method of roughening employed which produces very fine irregularities, as, for example, etching or fine sand blasting. The minute variations in luminosity from point to point would then not be detcctable in the optical image of the source, owing to the aberrations andoimperfect focusing of the lens or lenses.

Another object of my invention, therefore, is to provide a lamp adapted for use where a source of exceptional uniformity throughout its projected area is desired.

A further object of my invention is to provide an improved projection lamp which has greater power efliciency than those heretofore used.

Still another object of my invention is toprovide an improved projection lamp which will be small and compact andwhich will need less ventilation than lamps previously used.

In accordance with my invention, I provide a lamp in which a small rod or button of tungsten is heated to incandescence by electron bombardment from a filament associated therewith, and I place a third electrode in such a position relative to the tungsten electrode that it acts as a shield to prevent the arc from interfering with the light emitted by the incandescent button in the useful direction. In accomplishing this, it is necessary not only to hide the main body of the are from visibility from the direction of the optical system, but to strictly exclude the arc and all streamers from it or gaseous conduction of all kinds from the region in front of the incandescent electrode.

The novel features that I consider characteristic of my' invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with additional objects and advantages thereof, will best be understood from the following description of several embodiments thereof when taken in connection with the accompanying drawing in which Figure 1 is a central section of one form of my invention showing the nature and the relation of the various electrodes thereof,

Fig. 2 is a sectional view taken along the line 11-11 of Figure 1 with circuit connections to the lamp,

' Fig. 3 is a detail viw of a modified form of the invention, and

Figs. 4 and 5 show further modifications of my invention..

Referring more in detail to the drawing, wherein similar reference characters designate corresponding parts throughout, there is shown an envelope I which may be either highly evacuated erated on alternating current, as hereinafter described, the electrode 5 may actually become a cathode during a part of the cycle.

The cathode 3 may be connected through the conductor leads 9 to any suitable source of current, as, for example, a low voltage battery or, as is usually more convenient where an alternating current supply of electrical power is available, to the secondary Ila of transformer II, the primary of which is connected to the electric lighting mains. In the particular circuit shown in Figure 2, the anode 5 is maintained positive with respect to the cathode 3 by means of a bat tery I 4 connected between a mid -tap of the transformer secondary a and the anode 5 through conductor l3, while the shielding electrode I is maintained at a negative potential through conductor H by means of a battery or the like I5.

As shown in Figuresl and 2, the shielding electrode 1 comprises, a disc-like plate provided with an aperture is substantially centrally thereof and through which the tungsten button or anode 5 extends, the plate I being held in position by the conductor lead ll, which is heavy enough to serve as a satisfactory support, and lying in a plane nor- ,mal to that of the are between the electrodes 3 and 5 but outside of the arc path. The anode 5 extends through the aperture l9 and is supported in place by substantially radially extending wires 2! which are connected to the conductor lead l3, shown bent into a loop I31: and anchored on both sides of the loop, while the oathode 3 is supported in spaced relation to the anode 5 by meansv of the conductor leads 9. The anode 5 may be provided 'on the side toward the cathode with an enlarged head So, the purpose of which will be presently explained. Theother end of the anode 5,, or the end facing in the direction in which useful light is radiated (arrow A, Figure 1), has its radiating surface'increased in some suitable manner, as by making it concave,

grooved, or pitted. It is shown'in the drawing as having a number of small closely spaced pits, 5b,

deep relative to ,their diameters.

In Fig. 3, I have shown a modified form of the invention in which the shield I, which maybe of metal, such as tungsten, has beenjextended by the addition of an approximately spherical por-.

vtion 23, which serves to reflect back to the anode 5, a large fraction of the energy radiated by the back surface. This radiation serves no, useful purpose, and not only makes it necessary to supply more energy to the anode to keep it hot, and incidentally makes it necessary. to dissipate more heat through the surface of the bulb or envelope, but also makes more external shielding of the lamp necessary-to protect eyes of operators, and

keep down stray light. The reflector 23 can be either incorporated as part of the arc guard l,-

' or it may be a separate member. Since it would not have to be raised was high a temperature as the anode, it would not have to be made of as refractory a; metal as tungsten, but a more highly reflecting metal can be used. a I have also illustrated, inconnection with Fig.

3, a circuit which may be used for operation of the arc on alternating current, the alternating voltage being derived from an extra secondary winding 24a: of the power transformer 24. The cathode is heated by alternating current asin Fig. 2, but with the circuit. shown in Fig. 3, the

, electrode 5 would not be maintained continuously positive with respect to the filament 3, but alternately positive 'and'negative, During the period of areversed voltage, there would be a reverse current, but aside from the fact that the reversed are imparts less heat to the electrode 5 than an equal current in the direction which makes electrode 5 an anode, the reverse current is not objectionable. The magnitudes or, the arc current in the two directions would depend upon the voltage andon the resistance 18, upon the electron emission of the two electrodes in view of their respective sizes, temperatures, and materials, andln some measure on the form of the electric fleld as determined by the shapes and positions of all these electrodes. If the arc is operated on alternating instead of direct current,

the heat imparted to the electrode 5 will no longer I be continuous, but pulsating, each half cycle supplying a pulse of heat and those produced by the positive half cycles being greater than those produced by the negative half cycles. This irregular supply of heat means that the temperature of the anodecannot be maintained strictly constant. but herein lies the advantage of this form of lamp as compared with one of the incandescent filament type. Owing to the thickness of the electrode 501* its large mass compared with its radiating surface, it constitutes a large reservoir of thermal energyand cools down very slightly during the periods in which it is receiving less energy than it is radiating. Therefore, although there will be fluctuations in radiation, they will be small enough to be unobjection- I potential if ,it were simply allowed to float, or seek its own potential, or in other words, if it were not connected to the remainder of the circuit. This would mean simply omitting battery I5, resistance l6, and conductor l'l. The negatively charged guardl would still be efllcacious in conflning the arc to the back of the electrode 5. This makes the requirements for operating the lamp entirely on alternating current extremely simple, namely a transformer with suitable windings to supply the filament current and the; arc current, and a ballast resistance or reactor connected in circuit with the arc. The battery l5 has been included in the illustrations simply to show how such a battery or source of bias voltage would be connected if, for special reason, it should be'found necessary or desirable to maintainthe guard at a greater negativevoltage than that which it would assume it floating. If the lamp is to be operated with the shield I floating or insulated from the electrodes, theshield I would itself not need to be necessarily of conductthe arc to due side of the anode and to exclude from the optical system any light originating in the arc. From the foregoing explanations, it will be seen that the front surface of the anode 5 constitutes a source of light in which all the factors making for maximum radiation in the useful direction from a source of limited area can be brought to their optimum values. There are no dark holes or waste spaces, the anode may be raised, by regulating the value of the are curall rent, to as high a temperature as tungsten will stand without melting or excessive evaporation, and the roughening of the surface increases the specific brightness in a marked degree. The heat is, of course, generated at the back surface of the anode and radiated at the front, but the heat conductivity of the tungsten is so high that there would be only a small temperature drop between the two faces.

It would be possible to provide two cathodes, with a special shield between them, and operate the lamp like a full wave rectifier. The purpose of such an arrangement would be to insure equal impulses of heat during the two halves of the A. C. cycle, and thereby still further reduce temperature fluctuations on A. C. operation.

The annular opening around the anode would, of course, permit a certain amount of light from the arc to pass. Such light, however, would not get through the optical system if this is suitably designed. Only such light as is directed within the core subtended by the condensing lens 25 can enter the optical system, and if the mask 2? is slightly smaller than the image of the-anode, the mask will exclude any possible light which might come from the surrounding annular crack. In order to reduce the possibility of appreciable light entering the optical system from this source, the anode is shown as having an enlarged head So. Fig. 4 shows an alternative arrangement wherein the forward or pitted end of the anode is enlarged to, serve the same purpose. Another arrangement is illustrated in Fig. 5, in which a. mask 29 is provided in front of the anode 5 to hide the edges of the anode, around which some light from the arc might otherwise enter the optical system. In Fig. 5,'the mask 29 is shown as constituting part of the supportingstructure for the anode 5, but the maskmight equally well be an independent member. It would, in general, be mounted as close to the anode as feasible. The Wire or wires 2| which support the anode are preferably located on the side of the shield I away from the filament 3. This insures that the arc will not strike to the wires. porting wire carried the anode 5, it would have to be much heavier than the three radiating wires shown,'or of greater cross section than. the aggregate cross section of the three small wires, in order to provide an equally rigid support. The heat carried away by supporting wires is directly proportional to their cross section. Hence, a support of the form shown in the drawing is preferable to. a single wire support.

For certain optical arrangements, a circular source of light is not the most advantageous. It is obvious that the shape of the anode and the corresponding hole in the shield I may be made to suit the specific purpose. For example, if a long narrow slit is to be illuminated, it might be desirable to accomplish this by forming an image of the anode in the plane of the slit. For such purpose, the anode might advantageously be made rectangular and provided with a groove parallel to the longer axis.

The bottom of the groove, which would be the brightest part of the anode, would then be imaged on the slit.

In other applications, it may be desired to form an image of a definite shape, as for example a triangle, and of the best possible uniformity. In present practice, this is attained by special arrangements of lenses and a mask, whereas it can be accomplished very much more simply when using the lamp which is the subject of this If a single supfrom being visible from the mask 29 of Fig. 5, and adapting the shape and surface of the anode to correspond. This feature could beincorporated in a high vacuum lamp as well as in a gas-filled lamp.

Although I have shown and described several specific embodiments of my invention, I am fully aware that many modifications thereof and changes therein are possible. I therefore desire that my invention shall not be limited except insofar as'is necessitated by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. In an electrical discharge device, a pair of electrodes adapted to have an are maintained therebetween, and means in close proximity to only one of said'electrodes for preventing said arc from being visible fromone side of said last named electrode.

2. In an electrical discharge device, a pair of electrodes adapted to have an are maintained therebetween, and means'surrounding only one of said electrodes for preventing said are from being visible from one side of said last named electrode.

3. In an electrical discharge device, a pair of electrodes adapted to have an arc maintained therebetween, one of said electrodes having an enlarged head portion, and a third electrode of opaque material having an opening therein, said one electrode extending through said opening, and

said opening having a smaller diameter than plane normal to that of the arc for preventing the are from being visible from-the direction of useful light radiation.

5. In a lamp in which the source of useful light comprises a small body of refractory material adapted to be heated to incandescence by means of an electrical arc, a shielding member of opaque material surrounding said body in spaced relation to the arc path for preventing the arc the direction of useful light radiation.

6. In a projection lamp, a filamentary cathode, an anode adapted to be heated to incandescence by an electrical are from said cathode andv to serve as a source of useful'light, and a shield of opaque material surrounding said anode outside of the arc path for preventing said arc from being visible from the direction of useful light radiation. 1

'7. The invention set forth in claim 6 characterized in that theanode is roughened at the end thereof facing the direction of useful light radiation.

8. The invention set forth in claim 6 characterized in that the anode has one end thereof concave and that said concave end faces in the direction of useful light radiation.

9. The invention set forth in claim 6 characterized in that the anode is made of refractory material and characterized further in that the end invention, by suitably shaping the aperture in thereof facing in the direction of useful light radiation is concave to said direction.

10. The invention set forth in claim 6 characterized in that said anode is made of refractory material and characterized further in that said lamp comprises a gas filled envelope 12. The invention set forth in claim 6 characterized by the addition of a reflector on the are side of said shield for reflecting radiant energy emitted by the incandescent anode in directions other than the direction of useful light 5 radiation back onto said anode.

EDWARD W. KELLOGG. 

